Recently, there is an increasing interest in energy storage technology. For example, as applications of energy storage technology are expanded to mobile phones, camcorders, notebook PCs, and electric vehicles, efforts for research and development of energy storage technology are gradually materialized. Electrochemical devices are the most spotlighted field among the fields of energy storage technology, and interests are focused on the development of chargeable and dischargeable secondary batteries among the electrochemical devices. In particular, recently, in developing secondary batteries, research and development for design of novel electrodes and batteries are being carried out to improve capacity density and specific energy.
Since lithium secondary batteries among such secondary batteries have advantages of higher operation voltage and much greater energy density than conventional batteries using an aqueous solution (electrolyte), the lithium secondary batteries are widely used in various fields requiring energy storage technology.
A process of manufacturing a lithium secondary battery includes a process of forming an electrode active material layer on an electrode current collector. The process of forming the electrode active material layer includes coating active material slurry, in which electrode active material particles are dispersed in a binder solution, on the electrode current collector, and forming the electrode active material layer on the electrode current collector by removing the solution and water present in the active material slurry by drying the active material slurry coated on the electrode current collector.
FIG. 1 is a schematic diagram illustrating a conventional apparatus for coating electrode active material slurry, FIG. 2 is a partially enlarged view of a region I of FIG. 1, FIG. 3 is a partially enlarged view of a region II of FIG. 2, and FIG. 4 is a partially enlarged view of a region III of FIG. 2.
As shown in FIG. 1, a conventional apparatus 1 for coating electrode active material slurry, for performing a process of forming the electrode active material layer set forth above, includes: a supply roll 10 unwinding an electrode current collector E wound in a roll shape and continuously supplying the electrode current collector E in a predetermined process direction; a coating die 20 coating an active material slurry S, which is supplied from an external slurry supply source (not shown), on the electrode current collector E continuously moving in the process direction; a dryer 30 forming an electrode active material layer A on the electrode current collector E by drying the active material slurry S coated on the electrode current collector E; and a recovery roll 40 recovering the electrode current collector E, on which the electrode active material layer A is formed, in a roll state by winding the electrode current collector E.
Here, as shown in FIG. 2, the coating die 20 coats the active material slurry S on coating areas T of the electrode current collector E, the coating areas T being arranged at predetermined intervals. The coating die 20 is fixed and mounted at a predetermined position so as to face the coating areas T of the electrode current collector E, whereas the electrode current collector E is continuously moved along the process direction. Thus, when the active material slurry S ejected from the coating die 20 encounters the coating areas T of the electrode current collector E, inertial force I acts on the active material slurry S in an opposite direction to a moving direction of the electrode current collector E, that is, in an opposite direction to the process direction.
In addition, as shown in FIG. 2, the active material slurry S is selectively coated only on the coating areas T instead of being continuously coated throughout all regions of the electrode current collector E, and has a high coefficient of viscosity due to physical properties thereof. Thus, as shown in FIG. 2, in each of the coating areas T, viscous force V selectively acts on the active material slurry S coated on a balcony region B only in the opposite direction, selectively acts on the active material slurry S coated on a drag region D only in the process direction, and acts on the active material slurry S coated on a main coating region M in both the process direction and the opposite direction. Here, the balcony region B corresponds to a leading end of each of the coating areas T and refers to a region on which coating of the active material slurry S begins; the drag region D corresponds to a trailing end of each of the coating areas T and refers to a region on which coating of the active material slurry S is terminated; and the main coating region M corresponds to a middle portion of each of the coating areas T and refers to a region between the balcony region B and the drag region D.
As such, both the inertial force I and the viscous force V act on the active material slurry S coated on the balcony region B in the opposite direction. Thus, the active material slurry S coated on the balcony region B is biased in the opposite direction. However, since the main coating region M is in a location corresponding to the opposite direction with respect to the balcony region B, the active material slurry S coated on the balcony region B is supported by the active material slurry S coated on the main coating region M when biased in the opposite direction. Therefore, as shown in FIG. 3, the active material slurry S is coated in a bulging shape on the balcony region B.
The inertial force I acts on the active material slurry S coated on the drag region D in the opposite direction, and the viscous force V acts on the active material slurry S coated on the drag region D in the process direction. However, when the moving speed of the electrode current collector E is considered, since the magnitude of the inertial force I is generally greater than the magnitude of the viscous force V, a portion of the inertial force I remaining after canceled out by the viscous force V acts on the active material slurry S coated on the drag region D in the opposite direction. Thus, the active material slurry S coated on the drag region D is biased in the opposite direction. However, even though biased in the opposite direction, the active material slurry S coated on the drag region D, unlike the active material slurry S coated on the balcony region B, is not supported by the active material slurry S coated on the main coating region M. Therefore, as shown in FIG. 4, the active material slurry S is sharply coated on the drag region D such that the coating thickness thereof decreases along the opposite direction.
In addition, when coating of the active material slurry S on the electrode current collector E is terminated, that is, when the coating die 20 terminates the ejection of the active material slurry S, the ejection of the active material slurry S is gradually terminated over certain time instead of being terminated all at once. Thus, when the coating die 20 coats the active material slurry S on the drag region D, an ejection amount of the active material slurry S per unit time is gradually reduced such that the ejection of the active material slurry S onto the electrode current collector E may be terminated. Therefore, the active material slurry S is sharply coated on the drag region D such that the coating thickness thereof further decreases along the opposite direction.
The inertial force I acts on the active material slurry S coated on the main coating region M in the opposite direction, and the viscous force V acts on the active material slurry S coated on the main coating region M in both the process direction and the opposite direction. Thus, although the inertial force I causes the active material slurry S coated on the main coating region M to be biased in the opposite direction, the viscous force V causes the active material slurry S, which is biased in the opposite direction by the inertial force I, to be spread flat. Therefore, as shown in FIG. 2, the active material slurry S is coated relatively flat on the main coating region M.
To sum up the above descriptions, the active material slurry S is coated in a bulging shape on the balcony region B such that the coating thickness thereof is relatively higher than the coating thickness of the active material slurry S coated on the other regions, and the active material slurry S is sharply coated on the drag region D such that the coating thickness thereof is relatively lower than the coating thickness of the active material slurry S coated on the other regions.
As such, since the balcony region B and the drag region D, in which non-uniformity of the coating thickness of the active material slurry S occurs, may cause a problem in processability or the performance of a secondary battery when used in a manufacturing process of the secondary battery, the balcony region B and the drag region D are not used in the manufacturing process of the secondary battery and are discarded. Therefore, in the case of the conventional apparatus 1 of coating electrode active material slurry, since a non-uniform coating thickness region not used in a manufacturing process of a secondary battery, that is, a dead space is generated in the electrode current collector E, there is a problem of deterioration in economic efficiency and yield.
To solve the above problems, a method of mixing an additive with the active material slurry S has been conceived such that non-uniformity of the coating thickness of the active material slurry S does not occur in the balcony region B and the drag region D. However, when an additive is mixed with the active material slurry S, an overall composition of the active material slurry S needs to be changed according to a material of the additive. Therefore, the method of mixing an additive with the active material slurry S has a problem of deterioration in economic efficiency.